This invention relates to a process for recovering a usable population of fibers and fines from the waste sludge of a fiber production or fiber handling facility.
In particular, this invention provides a steam explosion process and a resulting product which permits the separation of a useful population of fibers and fines from the waste streams of fiber production processes such as a waste paper recovery operation or from the waste stream of a paper making process. The process is useful in fiber and fine recovery from white water waste streams from paper making operations and waste sludge from the de-inking and processing of waste paper. The present invention not only increases the amount of usable fibers and fines recoverable from the waste stream, but increases the fiber quality of the recovered fibers. Additional benefits include a concomitant reduction in the solid volume of the waste stream and increases the usable fiber content available from the initial raw material. The process steps of the present invention yield a fiber of improved quality suitable for making tissue products such as toilet and facial tissues, paper towels, and napkins.
Waste sludge from fiber production and paper making facilities typically contain numerous fibers and fines. In particular, efforts to recover the fibers and fines from the waste sludge streams of paper fiber recovery plants have been limited, in part, by the high ash content of the sludge. Conventional fiber screening techniques also retain the ash particulates. The high ash content renders the recovered fibers and fines undesirable for quality end products.
Paper is conventionally made by draining a low consistency dispersion of cellulose fiber pulp, fillers, and additives through a paper machine xe2x80x9cwirexe2x80x9d (essentially an endless mesh or sieve). A certain amount of solid material passes through the wire with the suspending water and is, thus, not retained in the wet paper web formed on the wire. The drained liquid suspension, known generally in the industry as xe2x80x9cwhite water,xe2x80x9d carries entrained solid material. White water from which the suspended particles have been removed is reused in the paper making process to the extent possible.
Obviously, wastepaper, if it can be recycled, is a much cheaper and environmentally friendly source of wood pulp for making paper. Before wastepaper can be reused as recycle material, however, the wastepaper must be de-inked. De-inking processes remove inks and coating materials from the wood fibers. Thus, when recycled fibers, as opposed to virgin pulps, are used in the paper making process, the drained liquid suspension will contain additional types of waste materials such as inks and hot melt adhesives.
Unused white water and de-inking effluents must be treated before being discharged from the paper mill. Treatment normally involves passing the effluent through a clarifier, prior to which flocculated are added to promote sedimentation of solid material suspended in the water. A biological treatment with microorganisms is also commonly performed to reduce the biological oxygen demand (BOD) of the liquid effluent before it is discharged. As can be expected, disposal of the unused white water and de-inked effluents results in costs to the paper making facility.
The sediment accumulated in the clarifier is a sludge composed of pulp fibers, fiber particles or fines, fillers, pigments, and other miscellaneous debris such as grit, sand, plastic particles, general dirt. Many of the sludge components are fillers, pigments and the like that were added to the pulp during the sheet-forming process for the purpose of producing desired properties in the finished paper. Such properties include proper surface, opacity, strength and brightness. For example, finely ground inorganic fillers, such as talc, certain clays, calcium carbonate, blanch fixed, and titanium dioxide may be added to papers to improve surface smoothness, whiteness, printability and opacity. Sizing agents, such as soaps, gelatins, and rosins (with alum), wax emulsions and starches, may be added to papers for improving resistance to penetration by liquids. In addition, coloring agents, such as acid, basic, direct and sulfur dyes and natural and synthetic pigments may be added for coloring purposes. Any of such products may ultimately end up in the clarifier as part of the sludge. In addition, because the clarifier is usually a large open air tank, other debris such as leaves, branches, insects, etc. Can also become part of paper sludge. The major constituents of the sludge, however, are generally fiber/fines and the inorganic fillers calcium carbonate and clay.
Most de-inking processes involve the use of flotation and washing. In de-inking processes, wastepaper is first washed and then pulled with dilute sodium hydroxide or surfactants in a pulpier tank to cause the fibers to swell and loosen the ink and coating material particles contained thereon. (These coating materials include the previously mentioned clays, talc, etc.) After pulling, the pulp stocks go through screening, cleaning, washing, floatation, and bleaching to further remove trash, stickiest, inks, ash, and short fiber fines. During the washing and floatation stages, most ash, stickiest, and short fiber fines are separated from the pulp stock. Thus, when the sludge comes from a mill using recycled waste paper, this sludge may also have accumulations of adhesives (otherwise known as xe2x80x9cstickiestxe2x80x9d), foreign bodies (such as pieces of plastic material or metal, otherwise known as xe2x80x9ccontrariesxe2x80x9d) in very small quantities, and other additives, such as those described above, that are used in the paper making process.
Normally, the sludge is drawn off from the clarifier at about 2.5 percent consistency (or xe2x80x9cpercent dry solids contentxe2x80x9d) and is then dewatered to a consistency of around 20 to 55 percent, for example, by means of rotary vacuum filters, screw presses, or belt presses. Dewatering reduces the weight of material going to the landfill and reduces the charges for landfill disposal because these are typically based on weight. Since the majority of the weight in the sludge comes from water, it behooves the sludge processor to remove as much water as possible. The dewatered sludge is in a semi-solid state and usually contains about 40 percent to about 80 percent by dry weight relatively fine wood fibers and from about 20 percent to about 60 percent inorganic (also referred to as xe2x80x9cashxe2x80x9d) and the additives mentioned above. The material typically is a crumbly, not very cohesive material that appears to be dry. At thirty percent consistency, most sludges are more like dry solids as opposed to a suspension or dispersion. Because of the non-cohesive character of sludge, the materials handling equipment for moving, storing and transporting are generally the same as for dry materials. Once in this state, the sludge is then capable of being collected and transported for disposal in landfills.
According to some sources, it is estimated that the amount of dry waste (waste sludge with substantially all of the residual water removed) produced due to paper processing exceeds 4.6 million tons per year. This sludge is produced by both paper making from virgin pulp and paper making from recycled fibers. A typical de-inking plant employing recycled fibers processes about 100 dry tons of waste paper into about 65 to about 80 dry tons of recycled (reusable) fiber. The remaining 20 to 35 tons of waste paper is unusable, and becomes part of the sludge produced by the deinking plant. After recycled fiber sludge is dewatered with various suitable dewatering devices, including, for example, a belt press or screw press, 100 dry tons of waste paper still produce from about 70 to about 120 wet tons of sludge which must be disposed.
Moreover, the sludge produced during the making of tissue from an integrated mill with a recycled fiber plant produces 10 times the amount of sludge produced during the making of tissue from virgin pulp. The typical virgin pulp tissue making process with a 200 ton capacity produces 10 tons of sludge per day while the typical tissue making with recycled fiber plant produces 100 tons of sludge per day.
Conventional methods for disposing of sludges include landfill, land spreading, composting and incineration. Landfill and land spreading sites are being depleted at an alarming rate, and the establishment of new sites is difficult due to environmental concerns. In addition, the cost associated with using landfills to dispose of sludge is constantly increasing. For example, paper manufacturers typically spend about $30/wet ton to send sludge to the landfill. Composting and incineration of sludge also raise environmental concerns. Some innovative sludge disposal techniques include processing the sludge into pellets for fuel or into lightweight aggregates for construction, pyrolysis, gasification, and incorporation into cements. However, these techniques generally require the use of complex methods and expensive equipment. In addition, attempts to recycle sludge to make paper have been unsuccessful because the process is inefficient due to drainage problems resulting from the presence of slow drying fines which tend to clog the wires and other equipment.
Due to the extremely large amounts of waste sludge generated from both the virgin pulp paper making and the recycled fiber paper making processes, new uses of sludge are needed in order to curtail the disposal problems presently being encountered. Some attempts have been made to create such uses.
The prior art sets forth basically five different approaches for utilizing sludge in useful products:
(1) Pelletizing the sludge using high pressure and binders where the sludge is dried before pelletizing. The pellets can be used as absorbents or chemical carriers, e.g., fertilizer. Alternatively, large diameter pellets or cylinders are used as fuel.
(2) Extracting the fibers or fillers from sludge in various ways to subsequently use the extracted material in a paper and/or ceramic product. These are both wet and dry processes.
(3) Mixing the sludge with other construction ingredients such as concrete or plastic to embody the sludge as reinforcing fibers or filler. Again these are both wet and dry processes.
(4) Direct molding of sludge into large shapes (i.e., large cross section) and drying. These products are construction blocks or boards and can be made using both wet and dry processes. Some of these can be fired to burn out the cellulosic and polymeric materials, leaving a ceramic product.
(5) Some sludges are formed into particulates or briquettes of various forms and sizes. These are subsequently carbonized to make an activated carbon product.
U.S. Pat. No. 4,303,019 to Haataja discloses the making of pellets by molding paper mill sludge blended with a fibrous reinforcing material. U.S. Pat. No. 5,215,625 to Burton discloses the making of products such as stepping stones, acoustic paneling, flower pots and planters, sculptures, shipping containers and packing materials, and the like, from waste products such as ink and waste slurry from pulp manufacturing. The incorporation of de-inking byproducts from wastepaper recycling. operations and pulp mill clarifier sludge into drywall and other gypsum-based building products is disclosed in U.S. Pat. No. 5,496,441 to Tran.
Attempts have also been made to recover and re-use the raw materials from paper mill waste sludge. For example, U.S. Pat. No. 5,478,441 to Hamilton, discloses a process for recovering such raw materials. U.S. Pat. No. 5,332,474 to Maxham discloses a process for producing a paper making filler product from the fiber fines/clay fraction of a pulp, paper, paperboard, or deinking mill waste solids.
While considerable prior art exists with regard to methods for handling, utilizing or recycling of sludge as outlined above, in actual fact there has been little commercial implementation. The majority of the materials classified as sludge from both deinking operations as well as conventional pulp and paper mills ends up in landfills or is discarded or disposed of in some other way. The major reason is that few of the many procedures available to convert sludges can produce products that have significant value.
The need to develop methods and processes that would use these waste materials, however, is growing. It is likely that environmental and regulatory pressures to recycle paper and paper products will increase. This will mean there will be more de-inking and recycling operations in the future. In addition, it is likely that the percentage of inorganic materials in recycled paper, such as calcium carbonate and clay, will increase for a number of reasons. For example, calcium carbonate improves the long term stability of printing papers because the alkalinity of the calcium carbonate reduces the rate of discoloration and embrittlement of the paper. Additionally, both calcium carbonate and clay are used to increase the opacity of paper. In printing items such as magazines or advertising supplements, addition of inorganic materials allows for reductions in the amount of fiber used for the paper. The purpose for the added inorganic material is to provide superior opacity. In addition to the improved opacity, incorporation of clay as a coating or filler and calcium carbonate as a filler also provide improvements to the surface of the paper such that the quality of the printing is improved. Furthermore, wood pulp, even though it is a renewable resource, is becoming more expensive. The cost of wood pulp currently exceed the cost of calcium carbonate or clay, and it, therefore, makes economic sense to include considerable amounts of these fillers in paper.
Steam explosion has been used to defiber paper material as described in U.S. Pat. No. 5,262,003 to Chupka et al. and in U.S. Pat. No. 4,312,701, which are both incorporated herein by reference. As further discussed in Chupka, non-fibrous contaminants subjected to the steam explosions have a reduced particle size and are more readily washed out of the fiber suspensions.
Steam explosion is also used in the defibration of wood chips as taught in U.S. Pat. No. 4,798,651 to Kokta, incorporated herein by reference. In Kokta, wood chips are chemically treated followed by steaming and explosive decompression to defibrate the chips.
In U.S. Pat. No. 4,163,687 to Mamers et al., a nozzle design is disclosed which is used in cellulosic defibration which increases usable fiber content from wood chips and results in less fiber damage.
In U.S. Pat. No. 5,262,004 to Gilbert, incorporated herein by reference, steam explosion is used as part of a chemical separation and recovering process of chemical preservatives from wood chips from previously treated wood.
U.S. Pat. No. 5,122,228 to Bouchette et al., incorporated herein by reference, discloses a steam explosion process which is used in the de-inking of waste paper. The temperature ranges used by Bouchette are reported to decrease particle size of contaminants and increase the ability to strip contaminants from the fibers.
Australian Patent Office Application AU-B-29615/92, corresponding to U.S. application Ser. No. 840,370 filed Feb. 24, 1992. now abandoned, discloses a steam explosion of mixed grades of high and low quality waste paper which have been delignified by alkaline digestion.
As will be seen from the description and illustrations to follow, none of the above-identified references discloses or anticipates the present invention directed to the process of treating waste sludge and waste liquor with steam explosion to increase the recoverable yield of fibers and fines from the waste stream. Further, none of the references teach or suggest that the useful fiber characteristics of an individual fiber can be improved for absorbent products by the steam explosion process.
The present invention recognizes and addresses some of the limitations of prior art fiber production in paper making processes which discard a large percentage of fibers and fines as an unsalvageable and/or non-useful waste material.
It is, therefore, a general object of the present invention to provide a process for recovering a useful population of fibers and fines from the waste stream of a fiber production, waste paper recycling, or paper making process. In carrying out the present process, it has been found that a steam explosion process may alter the morphology of fibers and fines contained within a waste stream, the altered morphology facilitating not only the separation of the fines from the waste stream, but improving their useful qualities in a paper product or paper making process.
The invention sets forth a process by which a supply of treated, recovered fibers and fines have superior fiber quality characteristics for absorbent products than similar untreated fibers and fines. The present invention enables both the volume and dry solid content of waste streams to be reduced by the recovery of additional fibers and fines from the waste stream. As a result, it is possible to increase the percentage of fibers and fines from the starting material. It is further possible to incorporate the increased percentage of fibers and fines into a finished paper product without lowering the quality of the finished product.
These and other useful objects of this invention are achieved by a process and resulting product which provides a waste sludge stream containing uncaptured fibers and fines from a fiber processing facility; subjecting the waste stream to an elevated temperature and pressure; explosively releasing the pressure from said waste stream; passing the now treated waste material through a filter recovery apparatus; and, separating a population of recovered fibers and a population of recovered fines from the waste stream.
Another aspect of the present invention concerns an absorbent structure comprising modified cellulosic fibers prepared by the process disclosed herein.
One embodiment of such an absorbent structure is a handsheet comprising the modified cellulosic fibers prepared by the process disclosed herein, wherein the handsheet is prepared by a wet laid process.